In recent years, steel sheet for automobile use has been made higher in strength for the purpose of improving fuel consumption by reducing the weight of the chassis. In steel sheet for fuel tanks as well, due to the reduction of weight of tanks and greater complexity of chassis design and, further, the locations where the fuel tanks are placed, fuel tanks are becoming more complicated in shape and superior formability and higher strength are being demanded. In the past, to satisfy both demands of better formability and higher strength, high strength IF (interstitial free) steel comprised of IF steel made of ultralow carbon steel to which carbonitride forming elements like Ti and Nb are added plus P, Si, Mn, and other solution strengthening elements has been developed.
However, when using high strength steel sheet for fuel tanks, there is the problem that the coach peel seam weld zone has a low tensile strength at a low temperature. That is, even if making steel sheet high in strength, there is the problem that the welded joint strength will not be high enough to match the increase in strength of the steel sheet. This is because a tank is produced by welding flange parts of upper and lower cup shaped parts and the seam weld zone of the tank is a coach peel shape as shown in FIG. 1 (where the cross-sectional shape is one where the flanges of the steel sheet are made to abut each other flat and seam welded, below, this weld zone also referred to as a “coach peel seam weld zone” or “coach peel weld zone”). In particular in the case of high strength steel sheet, the'stress easily concentrates, the toughness falls, and the tensile strength becomes lower. This becomes a concern in terms of the breakage resistance in the case where the fuel tank, an important part in safety, receives impact due to collision in a low temperature region.
Further, IF steel fixes the C, N, etc. by precipitation as carbides or nitrides of Nb or Ti, so there is the problem that the crystal grain boundaries become extremely clean and secondary work embrittlement easily occurs due to grain boundary fracture after forming. Further, in the case of high strength IF steel, the insides of the grains are strengthened by solution strengthening elements. The relative grain boundary strength falls remarkably, so there is also the problem of promotion of secondary work embrittlement.
Further, steel sheet free from the formation of corrosion products causing clogging of the filter and free from pitting corrosion with respect to gasoline and alcohol or organic acids which gasoline produces upon deterioration is also being sought. In response to these demands, in the past, steel sheet plated on its surface with Pb—Sn alloy, Al—Si alloy, Sn—Zn alloy, and Zn—Al alloy has been proposed and used. For this reason, the steel sheet used as the substrate has to be good in hot dip plateability by these alloys.
Among these problems, several methods have been proposed to avoid second work embrittlement (for example, see Japanese Patent Publication (A) No. 5-59491 and Japanese Patent Publication (A) No. 6-57373). For example, Japanese Patent Publication (A) No. 5-59491 proposes technology for avoiding the deterioration of the secondary work embrittlement resistance due to grain boundary segregation by reducing the P content as much as possible in Ti added IF steel and adding larger amounts of Mn and Si by that extent so as to obtain high strength steel sheet superior in secondary work embrittlement resistance. Further, Japanese Patent Publication (A) No. 6-57373 proposes the technology of using ultralow carbon steel sheet and adding B in addition to Ti and Nb to raise the grain boundary strength and improve the secondary work embrittlement resistance. In the technology described in this Japanese Patent Publication (A) No. 6-57373, the B content is optimized for the purpose of improving the secondary work embrittlement resistance and preventing an increase of the load at the time of hot rolling accompanying the delay in recrystallization of the austenite grains.
Further, several proposals have been made for the purpose of improving the weldability (for example, see Japanese Patent Publication (A) No. 7-188777, Japanese Patent Publication (A) No. 8-291364, and Japanese Patent Publication (A) No. 2001-288534). For example, the technology described in Japanese Patent Publication (A)
No. 7-188777 carburizes ultralow carbon steel to which Ti and/or Nb has been added at the time of annealing and forms a martensite and bainite structure at the surface layer to try to improve the spot weldability. Further, the technology described in Japanese Patent Publication (A) No. 8-291364 adds Cu to the ultralow carbon steel and broadens the heat affected zone at the time of welding so as to raise the strength of spot welded joints.
Furthermore, the technology described in Japanese Patent Publication (A) No. 2001-288534 is technology adding Mg to steel to form Mg oxides and/or Mg sulfides in the steel sheet and thereby make the weld zone and heat affected zone finer in grain by the pinning effect. ISIJ Journal, vol. 65 (1979), no. 8, p. 1232 discloses the technology of finely dispersing TiN in thick steel sheet to improve the toughness of the heat affected zone of the weld zone.
Furthermore, several technologies for improving the hot dip plateability of high strength steel sheet have been proposed (see Japanese Patent Publication (A) No. 5-255807 and Japanese Patent Publication (A) No. 7-278745). For example, in the hot dip galvanized high strength cold rolled steel sheet described in Japanese Patent Publication (A) No. 5-255807, the content of elements inhibiting hot dip plateability is limited, that is, the content of S is limited to 0.03 mass % or less and the content of P to 0.01 to 0.12%, while Mn and Cr are positively added as strengthening elements. Further, in the high strength galvannealed steel sheet described in Japanese Patent Publication (A) No. 7-278745 improves the hot dip galvanization ability by making the interrelationship between the Si content and Mn content within a specific range.
To improve the secondary work embrittlement resistance, high strength steel sheet superior in secondary work embrittlement resistance is provided by adding B and optimizing the balance of addition of Mn—P (Japanese Patent Publication (A) No. 2000-192188). Further, to improve the secondary work embrittlement resistance, the technology of adding B, Ti, and Nb has also been disclosed (Japanese Patent Publication (A) No. 6-256900). Furthermore, technology relating to a welding method for improving the tensile strength of the coach peel weld zone distinctive to a tank (Japanese Patent Publication (A) No. 2007-119808) and technology relating to high strength steel sheet for drawing and pressing use (Japanese Patent Publication (A) No. 2007-169739, Japanese Patent Publication (A) No. 2007-169738, Japanese Patent Publication (A) No. 2007-277713, and Japanese Patent Publication (A) No. 2007-277714) have also been disclosed.